1. Field of the Invention
The invention relates to a sampling circuit for sequential sampling of a broadband periodic input signal, there being a nonlinear component to which a pulsed-shaped sampling signal is supplied, by which sampling is activated so that an output signal is produced.
2. Description of Related Art
The detection of a broadband signal or a signal characteristic which changes quickly in time can only be done by means of real time sampling with very great technical effort or not at all. A typical application is fill level measurement by means of a pulsed radar, the technical problem lying in measuring the propagation delay of the radar signal very accurately. For distance measurement accuracy of 1 mm, accuracy of time measurement of roughly 6 ps is necessary. One possible alternative for real time sampling is sequential sampling. Sequential sampling manages with a much smaller sampling rate, but is limited solely to use for periodic signals. In contrast to real time sampling, only one sampling value is recorded per signal period. Then, a high speed signal to be sampled is reconstructed from a plurality of individual sampling points. Since this sampling process results in frequency conversion of the input signal, this process can also be interpreted as harmonic mixing.
One conventional implementation of a sampling circuit is shown in FIG. 1. An input signal to be sampled is routed via a transmitting and receiving line to two switching diodes 2, 3 arranged anti-parallel. The two switching diodes 2, 3 arranged anti-parallel are operated in the idle state by a respective DC bias applied first in the reverse direction. During the actual sampling process, a pulse-shaped sampling signal s1(t) and a sampling signal s2(t) which is inverse to it conductively switch the switching diodes 2, 3 so that the segment between the feed of the input signal uc(t) to be sampled and the two sampling capacitors 4, 5 connected downstream of the switching diodes 2, 3 become low-resistance for a short time. The charging portion on the sampling capacitors 4, 5, which originates from the pulse-shaped sampling pulses, can discharge via the resistors 6, 7. Under the assumption that the sampling capacitors 4, 5 are fully discharged between the two sampling processes, the sampling points of the sampled input signal arise fundamentally from the average voltage us(t) prevailing over the sampling capacitors 4, 5. If the relationship s1(t)=−s2(t) applies between the two sampling signals, the nodes 8, 9 can be regarded as virtual ground points with respect to these signals. Consequently, in the ideal case, the sampling signals on these nodes 8, 9 are mutually cancelled out so that feedthrough of the sampling signals in the intermediate frequency path and the signal path is stopped, by which balancing of the circuit is achieved.
The disadvantages in this conventional approach are, among others, the extreme demands on the phase and amplitude characteristic of the two sampling signals which are inverse to one another, as are necessary for operational balancing. To ensure balancing, the two sampling signals must be identical according to absolute value, and at the same time, must have phases turned exactly 180°; this allows generation of these signals to become accordingly complex and makes the operation of the circuit susceptible to component tolerances.